David E. Rothstein

An Exotic Tree Alters Decomposition and Nutrient Cycling in A Hawaiian Montane Forest

We evaluated the effects of the exotic tree Fraxinus uhdei on decomposition dynamics and nutrient turnover in a montane Hawaiian rainforest. We used reciprocal transplants of litterbags between forests dominated by Fraxinus and by the native Metrosideros polymorpha to distinguish between endogenous (litter quality) and exogenous (for example, microclimate, nutrient availability, microbial and invertebrate communities) effects of Fraxinus on mass loss and nutrient dynamics of decomposing litter. Fraxinus produced greater quantities of litter that was thinner, had higher N and P concentrations, and lower concentrations of lignin and soluble polyphenols. Microbes decomposing Fraxinus litter produced fewer enzymes involved in N and P acquisition and more of those involved in cellulose degradation. Differences in litter quality and microbial activity resulted in a strong effect of litter type on rates of mass loss, whereby Fraxinus litter decomposed and released nutrients at nearly twice the rate of Metrosideros litter (k = 0.82 versus 0.48), regardless of site of decomposition. Although site of decomposition had no effect on rates of litter mass loss, Fraxinus litter decomposed under a Fraxinus canopy mineralized approximately 20% less P after one year than Fraxinus litter decomposed under a Metrosideros canopy. Furthermore, Fraxinus litter decomposed under a Fraxinus canopy immobilized greater amounts of N and P in the early stages of decay, suggesting that the large amounts of N and P in Fraxinus litterfall have raised nutrient availability to decomposers in the forest floor. Greater immobilization of N and P under a Fraxinus canopy may act as a governor on rates of nutrient cycling, limiting the degree to which Fraxinus invasion accelerates N and P cycling in this system.

Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO3−deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO3−uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO3−additions equivalent to 30 kg N ha−1 year−1. We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used 15N to determine the kinetic parameters of NO3−uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0–3.5 g NO3−−N m−2 and determined the kinetic parameters of NO3−and NH4+uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO3−fertilization (range = non-detectable to 33 nmol NO2−g−1 h−1). Maximal rates of NO3−uptake in overstory trees also were low, ranging from 0.2 to 1.0 μmol g−1 h−1. In seedlings, the mean Vmax for NO3−uptake in fine roots (1 μmol g−1 h−1) was approximately 30 times lower than the Vmax for NH4+uptake (33 μmol g−1 h−1). Our results suggest that A. saccharum satisfies its N demand through rapid NH4+uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO3−.

Plant-available organic and mineral nitrogen shift in dominance with forest stand age

Studies of soil nitrogen (N) availability over stand development have almost exclusively focused on mineral N, yet we increasingly recognize that plants can take up organic N in the form of free amino acids at biologically important rates. We investigated amino-acid and mineral N availability along a 10-site chronosequence of jack pine stands, varying in age from 4 to 60 yr following wildfire. We measured free amino-acid N and mineral N in soil extracts; native proteolytic rates; net N mineralization rates; and microbial amino-acid consumption via a 15N leucine tracer assay in 6 of the 10 sites (4, 10, 18, 22, 46, and 55-yr-old). Amino-acid N was consistently low in the youngest sites (4-10 yr), increased rapidly in mid-aged sites (15-22 yr), and was highest in stand age 46. In contrast, mineral N exhibited a parabolic shape (R2 0.499; P < 0.0001), with the youngest site and the four oldest sites containing the highest amounts of mineral N. As a result, amino-acid N as a percentage of amino-acid N + mineral N was greatest in mid-aged stands (e.g., 67% in the 22-yr-old stand). We observed no trend in proteolytic rates across the chronosequence (P = 0.632). Percentage 15N tracer recovery was lowest in the extractable organic N pool for the 4, 10, and 18-yr-old sites, though only site age 10 was significantly different from the older sites. Percentage of recovery in the organic N pool was significantly positively related (R2 = 0.798; P < 0.05) to standing pools of amino-acid N. Overall, our results suggest that heterotrophic consumption, not production via proteolysis, controls soil free amino-acid availability. Higher microbial demand for free amino acids in younger vs. older sites likely results from greater microbial C and N limitation early in stand development due to the lack of fresh litter inputs. Since aminoacid N exceeds mineral N in a time period of stand development where jack pine growth rates and N demand are highest, we speculate that amino-acid N may be important to the N economy of these forests.

Amino acid uptake by temperate tree species characteristic of low- and high-fertility habitats

The relationship between inorganic nitrogen (N) cycling and plant productivity is well established. However, recent research has demonstrated the ability of plants to take up low molecular weight organic N compounds (i.e., amino acids) at rates that often rival those of inorganic N forms. In this study, we hypothesize that temperate forest tree species characteristic of low-fertility habitats will prefer amino acids over species characteristic of high-fertility habitats. We measured the uptake of 15N-labeled amino acids (glycine, glutamine, arginine, serine), ammonium (NH4+), and nitrate (NO3−) by four tree species that commonly occur in eastern North America, where their abundances have been correlated with inorganic N availability. Specific uptake rates of amino acids were largely similar for all tree species; however, high-fertility species took up NH4+ at rates more than double those of low-fertility species, rendering amino acid N relatively more important to the N nutrition of low-fertility species. Low-fertility species acquired over four times more total N from arginine compared to NH4+ and NO3−; high-fertility species acquired the most N from NH4+. Arginine had the highest uptake rates of any amino acid by all species; there were no significant differences in uptake rates of the remaining amino acids. Our results support the idea that the dominant species in a particular habitat are those best able to utilize the most available N resources.

Spring ephemeral herbs and nitrogen cycling in a northern hardwood forest: an experimental test of the vernal dam hypothesis.

Abstract In the late 1970s R.N. Muller and F.H. Bormann posited their ”vernal dam” hypothesis, stating that spring-ephemeral herbs in deciduous forests serve as a temporary sink for N when overstory trees are dormant, and then release this N later, in the summer, when the trees are active. This hypothesis has gained wide acceptance, yet two of its critical assumptions have never been experimentally tested: (1) that N taken up by spring ephemerals would otherwise be lost from the ecosystem, and (2) that N from senesced ephemeral tissues contributes to increased rates of summertime N mineralization. To test these assumptions, I quantified patterns of N cycling and loss from a set of paired plots, half of which served as controls and from half of which all spring-ephemeral plants were removed. There were no significant differences in NO3– leaching between plots with and without spring ephemeral vegetation. These results are consistent with the relatively low rates of N uptake by the dominant spring ephemeral, Allium tricoccum, and its apparent preference for NH4+, which is far less mobile in soil than NO3–. In addition, based on sequential sampling, I found that soil microorganisms took up 8 times as much N during the spring than did spring-ephemeral herbs (microbial uptake=3.19 vs. plant uptake=0.41 g N m–2), suggesting that microbial immobilization of N is the dominant sink for N during this season. Removal of spring ephemeral vegetation also had no effect on summertime rates of net N mineralization. Furthermore, the addition of spring ephemeral litter to soil+forest floor microcosms did not significantly increase rates of N mineralization in a laboratory incubation. Instead, this experiment demonstrated the overwhelming influence of forest floor litter in controlling the release of mineral N from these soils. Overall, neither assumption of the vernal dam hypothesis holds true in this ecosystem, where patterns of N cycling and loss appear to be dominated by microbial decomposition of forest floor material and soil organic matter.

Conversion of open lands to short‐rotation woody biomass crops: site variability affects nitrogen cycling and N2O fluxes in the US Northern Lake States

Short‐rotation woody biomass crops (SRWC) have been proposed as a major feedstock source for bioenergy generation in the Northeastern US. To quantify the environmental effects and greenhouse gas (GHG) balance of crops including SRWC, investigators need spatially explicit data which encompass entire plantation cycles. A knowledge gap exists for the establishment period which makes current GHG calculations incomplete. In this study, we investigated the effects of converting pasture and hayfields to willow (Salix spp.) and hybrid‐poplar (Populus spp.) SRWC plantations on soil nitrogen (N) cycling, nitrous oxide (N2O) emissions, and nitrate (NO3−) leaching at six sites of varying soil and climate conditions across northern Michigan and Wisconsin, following these plantations from pre conversion through their first 2 years. All six sites responded to establishment with increased N2O emissions, available inorganic N, and, where it was measured, NO3− leaching; however, the magnitude of these impacts varied dramatically among sites. Soil NO3− levels varied threefold among sites, with peak extractable NO3− concentrations ranging from 15 to 49 g N kg−1 soil. Leaching losses were significant and persisted through the second year, with 44–112 kg N ha−1 leached in SRWC plots. N2O emissions in the first growing season varied 30‐fold among sites, from 0.5 to 17.0 Mg‐CO2eq ha−1 (carbon dioxide equivalents). N2O emissions over 2 years resulted in N2O emissions due to plantation establishment that ranged from 0.60 to 22.14 Mg‐CO2eq ha−1 above baseline control levels across sites. The large N losses we document herein demonstrate the importance of including direct effects of land conversion in life‐cycle analysis (LCA) studies of SRWC GHG balance. Our results also demonstrate the need for better estimation of spatial variability of N cycling processes to quantify the full environmental impacts of SRWC plantations.

Soil fungal and bacterial responses to conversion of open land to short-rotation woody biomass crops

Short‐rotation woody biomass crops (SRWCs) have been proposed as an alternative feedstock for biofuel production in the northeastern US that leads to the conversion of current open land to woody plantations, potentially altering the soil microbial community structures and hence functions. We used pyrosequencing of 16S and 28S rRNA genes in soil to assess bacterial and fungal populations when ‘marginal’ grasslands were converted into willow (Salix spp.) and hybrid poplar (Populus spp.) plantations at two sites with similar soils and climate history in northern Michigan (Escanaba; ES) and Wisconsin (Rhinelander; RH). In only three growing seasons, the conversion significantly altered both the bacterial and fungal communities, which were most influenced by site and then vegetation. The fungal community showed greater change than the bacterial community in response to land conversion at both sites with substantial enrichment of putative pathogenic, ectomycorrhizal, and endophytic fungi associated with poplar and willow. Conversely, the bacterial community structures shifted, but to a lesser degree, with the new communities dissimilar at the two sites and most correlated with soil nutrient status. The bacterial phylum Nitrospirae increased after conversion and was negatively correlated to total soil nitrogen, but positively correlated to soil nitrate, and may be responsible for nitrate accumulation and the increased N2O emissions previously reported following conversion at these sites. The legacy effect of a much longer grassland history and a second dry summer at the ES site may have influenced the grassland (control) microbial community to remain stable while it varied at the RH site.

Recent land use change to agriculture in the U.S. Lake States: Impacts on cellulosic biomass potential and natural lands

Perennial cellulosic feedstocks may have potential to reduce life-cycle greenhouse gas (GHG) emissions by offsetting fossil fuels. However, this potential depends on meeting a number of important criteria involving land cover change, including avoiding displacement of agricultural production, not reducing uncultivated natural lands that provide biodiversity habitat and other valued ecosystem services, and avoiding the carbon debt (the amount of time needed to repay the initial carbon loss) that accompanies displacing natural lands. It is unclear whether recent agricultural expansion in the United States competes with lands potentially suited for bioenergy feedstocks. Here, we evaluate how recent land cover change (2008–2013) has affected the availability of lands potentially suited for bioenergy feedstock production in the U.S. Lake States (Minnesota, Wisconsin, Michigan) and its impact on other natural ecosystems. The region is potentially well suited for a diversity of bioenergy production systems, both grasses and woody biomass, due to the widespread forest economy in the north and agricultural economy in the south. Based on remotely-sensed data, our results show that between 2008 and 2013, 836,000 ha of non-agricultural open lands were already converted to agricultural uses in the Lake States, a loss of nearly 37%. The greatest relative changes occurred in the southern half that includes some of the most diverse cultivable lands in the country. We use transition diagrams to reveal gross changes that can be obscured if only net change is considered. Our results indicate that expansion of row crops (corn, soybean) was responsible for the majority of open land loss. Even if recently lost open lands were brought into perennial feedstock production, there would a substantial carbon debt. This reduction in open land availability for biomass production is closing the window of opportunity to establish a sustainable cellulosic feedstock economy in the Lake States as mandated by current Federal policy, incurring a substantial GHG debt, and displacing a range of other natural ecosystems and their services.

Impact of a Historical Fire Event on Pyrogenic Carbon Stocks and Dissolved Pyrogenic Carbon in Spodosols in Northern Michigan

Inventories of fire-derived (pyrogenic) C (PyC) stocks in soils remain incomplete for many parts of the world, yet are critical to reduce uncertainties in global PyC estimates. Additionally, PyC dynamics in soils remain poorly understood. For example, dissolved PyC (DPyC) fluxes from soil horizons, as well as the influence of historical fire events on these fluxes and soil PyC stocks remain poorly quantified. In this study, we examined stock and concentration differences in soil PyC and leached DPyC, respectively, between two forest types in the Great Lakes region (USA): (1) a red pine (Pinus resinosa) forest planted after the site had experienced post-logging slash burning in the late 19th century (100yr-burned site), and (2) a sugar maple (Acer saccharum) forest that showed no evidence of burning in the past 250 years (unburned site). We hypothesized that the 100yr-burned site would have greater PyC stocks and concentrations of DPyC compared to the unburned site. We measured PyC in soil, as well as DPyC in soil water leaching from O and E horizons following a spring snowmelt event in both 100yr-burned and unburned sites. Additionally, we measured DPyC drained from B horizons in 100yr-burned site. In organic horizons, PyC stocks were 1.8 (Oi) and 2.3 (Oe) times greater in the 100yr-burned site than in the unburned site. Contrary to our initial hypothesis, DPyC concentrations did not differ between sites. On average, DPyC leached from all sites contributed 3.11±0.27% of the total dissolved organic carbon pool. In the 100yr-burned site, a significant decline in concentrations of DPyC leaving the B horizon was attributed to the immobilization of this C pool in the Al and Fe oxides-rich subsoil. Even though PyC stock in O horizons was higher in 100yr-burned than in unburned site, our results did not support our initial hypothesis that the 100yr-burned site would have greater DPyC concentrations than the unburned site, suggesting that any differences in DPyC resulting from a single fire event are either not detectable after >100 years post-burn, and/or that the release of DPyC is a continuous, long-term process resulting from the degradation of historically accumulated PyC.

Methodology for an Integrative Assessment of China's Ecological Restoration Programs

While research projects have been conducted to examine the impacts and effectiveness of Chinas ecological restoration programs, few of them represent integrated, systematic efforts. The objective of this chapter is thus to articulate and outline a methodology for an integrative assessment, which, we believe, should embrace both the environmental and socioeconomic changes and engage investigations at multiple scales. Further, these investigations should be pursued through interdisciplinary collaboration with expertise from ecology, economics, hydrology, and geospatial, climate, and land change sciences. We argue that the deployment of geospatial capability, the use of longitudinal data, and the connection between science and policy should be the hallmarks of an integrative assessment. We also describe our general approach and specific models to quantify the environmental and socioeconomic impacts induced by implementing the restoration programs, and address the issue of how to overcome the challenges in generating the data needed for executing various empirical tasks. We hope that the adoption and application of this methodology will make a valuable contribution to a more robust and timely assessment as well as implementation of the ecological restoration programs in and outside of China.